NAIROBI - About 50 African Union peacekeepers based in Somalia have died in clashes with militants linked to al-Qaeda over the past two weeks, suggesting a dramatic escalation in the fight for the Somali capital, Mogadishu.

The high death toll raises questions as to whether Burundi and Uganda, the two nations contributing the bulk of troops to the force, known as AMISOM, will remain committed to keeping their soldiers in Somalia in the months and years ahead.

The 8,000-member force is widely considered to be the main bulwark preventing Somalia's al-Shabab militants from ousting the nation's weak transitional government. An additional 4,000 troops are scheduled to arrive over the next several months.

The toll, reported by diplomats and U.N. security officials, is far higher than the handful of deaths confirmed so far by AMISOM, which appears to be concerned about political backlashes in Burundi and Uganda. Opposition leaders in Uganda for months have urged their government to pull its troops out of Somalia after al-Shabab orchestrated bombings in Uganda's capital, Kampala, last year during the World Cup, killing more than 70 fans. It was the group's first international attack.

The Associated Press, citing two unnamed diplomats in Kenya's capital, Nairobi, reported a death toll of 53 peacekeepers since Feb. 19, when a major offensive against the Islamist militants began.

Two U.N. security officials, speaking on the condition of anonymity because of the sensitivity of the matter, said that as many as 50 peacekeepers have been killed. Most of the peacekeepers killed over the past two weeks were Burundian.

The same U.N. sources said that as many as 500 al-Shabab fighters were believed killed over the past two weeks. AMISOM spokesmen declined to discuss the deaths.

"I am not in a position to comment," said Gaffel Nkolokosa, an AMISOM spokesman in Nairobi.

Maj. Barigye Bahuko, AMISOM's spokesman in Mogadishu, also declined to comment, saying a news conference on the matter was scheduled for Saturday in Nairobi.

In recent weeks, AMISOM has said it has retaken large swaths of the capital controlled by al-Shabab. The militants have retaliated with force, including a suicide bombing last month at a police base that killed at least seven people. The attack came a day after AMISOM said it had struck a major blow against al-Shabab by dismantling a network of tunnels used to funnel fighters into government-controlled areas.

Somalia has not had a functioning central government since 1991, when the collapse of Mohamed Siad Barre's government plunged the Horn of Africa nation into civil war.